Polytetrafluoroethylene molded articles coated with fused fluoropolymer resin

ABSTRACT

The present invention relates to a molded article of polytetrafluoroethylene or modified polytetrafluoroethylene having a fluoropolymer resin coating, the coating comprising a heat-flowable tetrafluoroethylene copolymer wherein the surface of the coated article has a reduced roughness compared to the molded article prior to coating. The coating for the molded article is preferably a fused powder, most preferably formed by electrostatically applying a fluoropolymer powder resin to the molded PTFE article. In a preferred embodiment, the fluoropolymer powder resin comprises a mixture of heat-flowable tetrafluoroethylene copolymer powder and a polytetrafluoroethylene that has a temperature of crystallization of at least 305° C. and a heat of crystallization of at least 50 J/g. The surfaces of the articles are smoother than the original articles so that they resist adhesion of chemical contaminants and have applicability for chemical containers and transport pipes in the rigorously clean environment of the semiconductor industry.

FIELD OF INVENTION

[0001] The present invention relates to molded PTFE articles for use ascontainers for high purity chemicals, typically in the area ofsemiconductor manufacturing, or as a lining material for transportpipes.

BACKGROUND OF THE INVENTION

[0002] Because of the excellent heat and chemical resistance ofpolytetrafluoroethylene (hereafter PTFE) and modifiedpolytetrafluoroethylene, articles formed from these polymers areextensively used in transport pipes that handle corrosive or hightemperature fluids or as corrosion resistant lining material for tanks,machines, equipment, and the like.

[0003] However, generally, the surface of PTFE molded articles is notsufficiently smooth. This is a cause for concern when such articles areused as containers for high purity chemicals or linings for transportpipes, since contaminants tend to adhere to the surface and aredifficult to remove even after attempts are made to clean the articles.In particular, the removal of contaminants has become a rigorousrequirement in areas requiring sophisticated physical properties,particularly in semiconductor fabrication or the like.

BRIEF SUMMARY OF THE INVENTION

[0004] The present invention provides for a molded article ofpolytetrafluoroethylene or modified polytetrafluoroethylene having afluoropolymer resin coating, the coating comprising a heat-flowabletetrafluoroethylene copolymer wherein the surface of the coated articlehas a reduced roughness as compared to the molded article prior tocoating. The coating for the molded article is preferably a fusedpowder, most preferably formed by electrostatically applying afluoropolymer powder resin to the molded PTFE article. In a preferredembodiment the fluoropolymer powder resin comprises a mixture ofheat-flowable tetrafluoroethylene copolymer powder and apolytetrafluoroethylene powder that has a temperature of crystallizationof at least 305° C. and a heat of crystallization of at least 50 J/g.

[0005] The invention further provides for a process for forming a smoothsurface on a molded article of polytetrafluoroethylene or modifiedpolytetrafluoroethylene by coating the article with a heat-flowablefluoropolymer powder resin and heating the coated article to form afused fluoropolymer resin coating wherein the surface of the coatedarticle has a reduced roughness compared to the molded article prior tocoating.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0006]FIG. 1 is a three-dimensional profile display of the surface of amolded PTFE article (Example 3) coated with a mixture of PFA powder andlow molecular weight PTFE powder in accordance with the presentinvention, measured using a laser type three-dimensional surfaceroughness measuring instrument.

[0007]FIG. 2 is a three-dimensional profile display of the surface of amolded PTFE article (Comparative Example 1) without a heat-flowablecoating measured using a laser type three-dimensional surface roughnessmeasuring instrument.

DETAILED DESCRIPTION OF THE INVENTION

[0008] PTFEs, because of their excellent characteristics such as heatresistance and chemical resistance, have been known to exhibitsatisfactory performance when used as containers for high puritychemicals. Previously, they were used with no modification. However inrecent years, in high technology industries, particularly, insemiconductor manufacturing areas, there is a stringent requirement forremoving contaminants, so that the existing PTFE alone can no longersatisfy the requirement. The present invention satisfies the need for amolded article having the heat and chemical resistance properties ofPTFE and in addition having a smoother surface that resists adhesion ofcontaminants to the surface. Specifically, a molded PTFE article with asmoother surface can be prepared by electrostatically powder coating thesurface of a PTFE molded article with a fluoropolymer powder resin. Thefluoropolymer powder resin comprises a heat-flowable tetrafluoroethylenecopolymer. Preparation of an article with this electrostatic coatingpermits the entire surface of the article, regardless of configuration,to be covered with heat-flowable fluoropolymer powder resin, therebyconsiderably improving the surface smoothness of the moldedpolytetrafluoroethylene article.

[0009] By smoother surface is meant that the surface of the coatedarticle has a reduced roughness as compared to a molded article prior tocoating.

[0010] % Reduced Roughness is Calculated by the Following Equation (1):$\begin{matrix}{\frac{{{{Surface}{\quad \quad}{Roughness}\quad \left( {{uncoated}\quad {article}} \right)} - {{Surface}\quad {Roughness}\quad \left( {{coated}\quad {article}} \right)}}{\quad \quad}}{{Surface}\quad {Roughness}\quad \left( {{uncoated}{\quad \quad}{article}} \right)} \times 100} & (1)\end{matrix}$

[0011] The coated articles of the present invention preferably have asurface roughness that is reduced by at least 25%, more preferably by atleast 50% and most preferably by at least 75% as compared to a moldedarticle prior to coating. The coated surface of the molded articles ofthis invention have a center line average roughness (R_(a)) of less than1.5 micrometers, preferably less than 1.0 micrometer, and mostpreferably less than 0.5 micrometer.

[0012] Molded PTFE

[0013] PTFE molded articles, substrates in this invention, are moldedarticles obtained by molding tetrafluoroethylene homopolymer (PTFE) ormodified polytetrafluoroethylene. By modified PTFE is meant that PTFEcontains a small amount of comonomer modifier which improvesfilm-forming capability during baking (fusing), such as perfluoroolefin,notably hexafluoropropylene (HFP) or perfluoro(alkyl vinyl ether),notably wherein the alkyl group contains 1 to 5 carbon atoms, withperfluoro(propyl vinyl ether) (PPVE) being preferred or fluoroalkylethylene or chlorotrifluoroethylene. The amount of such modifier will beinsufficient to confer melt-fabricability to the PTFE, generally beingno more than 1 wt %. The PTFE, also for simplicity, can have a singlemelt viscosity, usually at least 1×10⁹ Pa·s, but a mixture of PTFEshaving different melt viscosities can be used to form the moldedfluoropolymer component.

[0014] Heat-Flowable Fluoropolymer

[0015] Heat-flowable fluoropolymer resins that can be used in thisinvention are copolymers of tetrafluoroethylene and other comonomersthat melt and liquify to flow at temperatures above their meltingpoints. Examples of such copolymers include copolymers oftetrafluoroethylene and perfluoro(alkyl vinyl ether) (PFA) such asperfluoro(propyl vinyl ether), and copolymers of tetrafluoroethylene andhexafluoropropylene (FEP). “Heat-flowable” as used herein means that thefluoropolymer resin means that particles of the fluororesin will flowand fuse together upon heating above their melting temperature.

[0016] Preferred among the heat-flowable fluoropolymer resins that coatthe surface of PTFE molded articles, is PFA powder that is a crystallinecopolymer powder of tetrafluoroethylene and perfluoro(alkyl vinylether), the copolymer powder having a perfluoro(alkyl vinyl ether)content in the copolymer of 1-10% by weight. Suitable perfluoro(alkylvinyl ether) comonomers are perfluoro(propyl vinyl ether),perfluoro(ethyl vinyl ether), and perfluoro(methyl vinyl ether).

[0017] The copolymer powder is a heat-flowable fluoropolymer resin thatpermits melt molding such as melt extrusion molding, injection moldingand the like, and preferably has a melt flow rate (MFR) at 372° C.±1° C.of 0.5-500 g/10 minutes, more preferably 0.5-50 g/10 minutes.

[0018] Another useful heat-flowable fluoropolymer resin is FEP powder,which is a copolymer powder of tetrafluoroethylene andhexafluoropropylene where the hexafluoropropylene content in thecopolymer is 10-15% by weight of the copolymer powder. This copolymerpowder is a heat-flowable fluoropolymer resin that permits melt moldingsuch as melt extrusion molding, injection molding, or the like.

[0019] Low Molecular Weight PTFE

[0020] The above copolymer powders e.g., PFA or FEP can be used alone,but the use of a mixture with a specific low molecular weight PTFE canfurther improve surface smoothness. Such a PFA powder that is mostsuitable for the objectives of the invention is a mixture of a PFApowder and a polytetrafluoroethylene powder having a temperature ofcrystallization of at least 305° C. and a heat of crystallization of atleast 50 J/g. The average particle size of low molecular weight PTFE isless than 100 micrometers, and preferably 2-20 micrometers. A PFAcomposition containing low molecular weight PTFE for use as a moldingcomposition is previously described in U.S. Pat. Nos. 5,473,018 and5,603,999. Alternatively, the low molecular weight PTFE may be added inthe form of dispersion wherein the average particle size is 0.05micrometer to 1 micrometer.

[0021] Use of a conventional PTFE having physical properties other thanthe low molecular weight PTFE described above as a component will makeit difficult to achieve a smooth coating onto a substrate material byelectrostatic coating, failing to give a PTFE molded article withexcellent surface smoothness. In the above composition, the amount ofPTFE to be incorporated is 0.01-50% by weight, particularly 0.01-4% byweight with respect to the composition. Using too little PTFE reducesthe effect of improving surface smoothness, while using a levelexceeding 50% by weight produces a poor coating onto the substrate.

[0022] Electrostatic Powder Coating Procedure

[0023] The present invention calls for electrostatically powder coatingthe surface of a PTFE molded article substrate with a heat-flowablefluoropolymer resin e.g., PFA powder or an FEP powder for improvedsurface smoothness. PTFE molded articles, because of their excellentwater repellency and oil repellency features, are difficult tosurface-coat. It is extremely difficult to coat the surface of a PTFEmolded article with PFA or FEP by an impregnation coating or similartechniques. Also, there are limitations on the available configurationsof coatable PTFE molded articles even when the PTFE molded article ispretreated to make the surface of the molded article more receptive tothe coating. However, it has been discovered that use of electrostaticcoating of a heat-flowable fluoropolymer powder resin, e.g., PFA powderor FEP powder, readily provides an excellent coating and removesprevious limitations with respect to the configuration of the PTFEmolded article to be coated. Electrostatically powder coating using aPFA powder or a FEP powder permits controlling the thickness of the filmgenerated on the surface of a PTFE molded article, making it possible toprovide a film thickness of 100 μm or below. Thin films can produce asmooth surface on large articles of molded PTFE. The use of thin filmcoatings permits economic fabrication of large articles producingsurfaces that can resist adhesion of contaminants.

[0024] There are no particular limitations as to the method ofelectrostatically powder coating the surface of a PTFE molded articlewith a heat-flowable fluoropolymer resin. Use of any conventionalelectrostatic powder coating method/equipment is suitable for theapplication of the powder to produce smooth, coated, molded PTFEarticles of this invention.

[0025] A PTFE molded article of this invention, obtained by coating thesurface with a heat-flowable fluoropolymer resin retains the excellentphysical properties of fluororesins, such as heat resistance andchemical resistance, while exhibiting excellent surface smoothness. Thesurface smoothness of the article substantially suppresses contaminationin fluid handling equipment and therefore may be adapted to a broadrange of applications in the area of precision industry materials suchas in semiconductor processing areas.

Test Methods

[0026] Temperature of Crystallization

[0027] A Perkin Elmer differential scanning calorimeter DSC Model 7 isused to determine Temperature of Crystallization. A 5 mg sample isweighed in a dedicated aluminum pan, crimped by means of a dedicatedcrimper, mounted in a DSC instrument, and the sample is heated from 200°C. to 380° C. at 10° C./min. After the sample is held one minute at 380°C., it is cooled from 380° C. to 200° C. at 10° C./min, producing acrystallization curve from which the crystallization peak temperature isobtained as the temperature of crystallization (Tc).

[0028] Heat of Crystallization

[0029] The heat of crystallization (Hc) is obtained from thecrystallization curve by connecting with a straight line the point wherethe curve departs from the base line to the point at which the curvereturns to the base line before and after a given crystallization peakand measuring the peak area enclosed.

[0030] Surface Roughness

[0031] The center line average roughness (Ra) of the surface of PTFEmolded articles is measured using a laser type three-dimensional surfaceroughness measurement instrument (manufactured by Laser Tex, Model1LM21).

EXAMPLES

[0032] The present invention is now more specifically described usingthe examples below. The compositions, physical properties, andmanufacturing processes and the like, of the fluoropolymer resins usedas substrates and as coatings in these examples and comparative examplesare given below.

[0033] Fluoropolymer Resins

[0034] (1) PTFE Powder:

[0035] Average Particle Size 30 μm

[0036] “Teflon 170-J” (Registered trademark, Mitsui DuPontFluorochemicals KK)

[0037] (2) PFA Powder:

[0038] Average Particle Size 28 μm

[0039] Manufactured by Mitsui DuPont Fluorochemicals KK

[0040] Copolymer of tetrafluoroethylene and perfluoro(propyl vinylether) (PFA)

[0041] [perfluoro(propyl vinyl ether) content 3.6 wt %]

[0042] (3) FEP Powder:

[0043] Average Particle Size 32 μm

[0044] Manufactured by Mitsui DuPont Fluorochemicals KK

[0045] Copolymer of tetrafluoroethylene and hexafluoropropylene (FEP)

[0046] [hexafluoropropylene content 12.0 wt %]

[0047] (4) Low Molecular Weight PTFE Powder

[0048] Average Particle Size 2-20 μm

[0049] Temperature of Crystallization 305° C. or higher

[0050] Heat of Crystallization 50 J/g or higher.

[0051] PTFE Molded Article

[0052] PTFE molded articles are formed by filling a mold, 170 mmlong×170 mm wide×15 mm thick with 350 g of PTFE powder (1) andcompressing at a pressure of 150 kg/cm² and a compression speed of 10mm/min until a thickness of 4 mm is reached. After having reached thedesired thickness, the pressure is held 1 minute at 150 kg/cm², followedby deaerating to give a PTFE pre-molded article.

[0053] The resulting PTFE pre-form molded article is placed on a metalplate and sintered according to the following temperature cycle.

[0054] (i) Heating from room temperature to 370° C. in 2 hours.

[0055] (ii) Holding at 370° C. for 1 hour.

[0056] (iii) Cooling from 370° C. to 100° C. at the rate of −30° C./hr.

[0057] (iv) On reaching 100° C., the sample is removed from the oven andcooled to room temperature.

[0058] A PTFE molded article (sheet), 170 mm×170 mm×4 mm thickcomprising 350 g of PTFE powder is produced.

Examples 1 and 2

[0059] A heat-flowable fluoropolymer resin of PFA (2) or FEP (3) powder,as indicated in Table 1, is electrostatically spray coated onto a moldedPTFE sheet prepared as described above. The sheet is grounded, 25 cmaway. Coating is conducted at an electrical voltage of 10 kV (negative)and a rate of discharge of 50 mg/min using an electrostatic powdercoater (manufactured by Onoda Cement KK; GX-200T) and an electrostaticpowder coating gun (Onoda Cement KK GX-107). The applied powder weightis uniformly 2.2 g, thereby generating a 100 μm coated thickness aftersintering. The coating environment is at a temperature 25° C. and amoisture level of 60% RH. The coated PTFE sheet is sintered in a forcedair circulation oven at 370° C. for 30 minutes and then cooled to roomtemperature producing a molded PTFE article coated with fusedfluoropolymer resin of heat-flowable PFA or FEP.

[0060] The center line average roughness (Ra) of the surface of theresulting fused fluoropolymer coated PTFE sheet is measured using alaser type three dimensional surface roughness measurement instrumentand the results are shown in Table 1.

Example 3

[0061] A molded PTFE sheet prepared as described above iselectrostatically powder coated in a manner similar to that ofExample 1. However, in this example the powder is a mixture of 99% byweight of the PFA powder (2) and 1% of a low molecular weight PTFEpowder (4) having a temperature of crystallization of at least 305° C.and heat of crystallization of at least 50 J/g, thereby obtaining afused fluoropolymer resin coated PTFE sheet.

[0062] Center line average roughness of the surface of the resultantPTFE sheet coated with a PFA containing a low molecular weight PTFE ismeasured using a laser type three dimensional surface roughnessmeasurement instrument. The results are given in Table 1. Athree-dimensional profile obtained using this apparatus is shown in FIG.1.

Comparative Example 1

[0063] A molded PTFE sheet prepared as described above but not having afused fluoropolymer powder resin coating, e.g., without anymodification, is subjected to measurement of its surface center lineaverage roughness (Ra) using a laser type three dimensional surfaceroughness measurement instrument. The results are given in Table 1. Athree-dimensional profile obtained using this apparatus is also given inFIG. 2. TABLE 1 Example 1 Example 2 Example 3 Comp.Ex.1 SubstrateMaterial PTFE PTFE PTFE PTFE Molded article Molded article Moldedarticle Molded article Fluoropolymer resin Powder PFA (wt %) 100 — 99 —FEP (wt %) — 100 — — Low molecular wt — — 1 — PTFE (wt %) Method ofcoating Electrostatic powder Electrostatic powder Electrostatic powder —coating coating coating Center line average roughness (μm) 0.33 0.250.21 2.15

[0064] The results of Table 1 (FIGS. 1 and 2) clearly show that incontrast to the surface of the unmodified PTFE molded articles havingrough surfaces with indentations and protrusions, the fusedfluoropolymer resin coated PTFE molded articles of this inventionexhibit substantially improved surface smoothness. The coated moldedarticles of Examples 1, 2 and 3 with average surface roughness of 0.33,0.25 and 0.21 respectively have a roughness that is reduced by 85%, 88%and 90% respectively as compared to the molded article with no coatingof Comparative Example 1.

What is claimed is:
 1. A process for forming a smooth surface on amolded article of polytetrafluoroethylene or modifiedpolytetrafluoroethylene by coating said article with a heat-flowablefluoropolymer powder resin and heating the coated article to form afused fluoropolymer resin coating, wherein the surface of the coatedarticle has a reduced roughness compared to the molded article prior tocoating.
 2. The process of claim 1 wherein said heat-flowablefluoropolymer resin comprises a tetrafluoroethylene copolymer.
 3. Theprocess of claim 1 wherein said fluoropolymer resin coating comprises acopolymer of tetrafluoroethylene and perfluoro(alkyl vinyl ether) (PFA).4. The process of claim 1 wherein said fluoropolymer resin coatingcomprises a copolymer of tetrafluoroethylene and hexafluoropropylene(FEP).
 6. The process of claim 1 wherein said fluoropolymer resincoating comprises a mixture of heat-flowable tetrafluoroethylenecopolymer and a polytetrafluoroethylene that has a temperature ofcrystallization of at least 305° C. and a heat of crystallization of atleast 50 J/g.
 7. The process of claim 1 wherein said coating is formedby electrostatically applying said fluoropolymer powder resin to saidmolded PTFE article.
 8. The process of claim 1 wherein the surface ofthe coated article has a roughness that is reduced by at least 25% ascompared to the molded article prior to coating.
 9. The process of claim1 wherein the surface of the coated article has a roughness that isreduced by at least 50% as compared to the molded article prior tocoating.
 10. The process of claim 1 wherein the surface of the coatedarticle has a roughness that is reduced by at least 75% compared to themolded article prior to coating.
 11. The process of claim 1 wherein thecoated surface has a center line average roughness (R_(a)) of less than1.5 micrometers.
 12. The process of claim 1 wherein the coated surfacehas a center line average roughness (R_(a)) of less than 1.0 micrometer.13. The process of claim 1 wherein the coated surface has a center lineaverage roughness (R_(a)) of less than 0.5 micrometer.
 14. The processof claim 1 wherein the thickness of the fused fluoropolymer resincoating is 100 μm or less.